<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>盒子模型</title>
  <style>
    #div1 {
      width: 400px; /* 宽度: 400像素; 默认是内容展示区域的宽度 */
      height: 300px; /* 高度: 300像素; 默认是内容展示区域的高度 */
      background-color: #ffff00;
      padding: 30px; /* 内边距: 30像素 */
      box-sizing: border-box; 
      border: 20px solid #ff00ff; /* 边框: 20像素 */
      margin: 30px auto; /* 外边距: 20像素 */
    }
  </style>
</head>

<body>
  <div id="div1">
    A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A 
  </div>

  <!-- <div id="div2">
    A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A A
  </div>

  <span>
    B B B B B B B B B B B B B B B B
  </span>

  <span>
    B B B B B B B B B B B B B
  </span> -->
</body>
</html>